This invention relates to acoustic surface wave apparatus and more particularly to a novel acoustic surface wave transducer capable of launching and receiving surface waves in a body of piezoelectric material with a unidirectional response characteristic.
Surface wave apparatus, as is well known in the art, generally consists of an acoustic surface wave propagation medium and one or more electromechanical transducers for launching and receiving surface waves into and from the medium. The most common type of transducer is called an interdigital transducer and consists of one or more sets of interleaved metal electrodes, deposited on the surface of the piezoelectric material with a spacing of 1/2 wavelength of the frequency at which the transducer is to operate. One finger from each pair is electrically interconnected by a first bus rail which forms a first terminal of the transducer and the remaining finger in each pair is electrically interconnected by a second bus rail which forms the second terminal of the transducer. When a single phase electrical signal is supplied to such a transducer the resulting surface wave can propagate in two directions. Such a transducer therefore has a minimum conversion loss of 3 db because half of the mechanical power is excited in each direction. Since this loss exists whether converting from an electrical signal to an acoustic surface wave or from an acoustic surface wave to an electrical signal the minimum net terminal loss for a two transducer acoustic surface wave apparatus is 6 db.
In the prior art it has been common to make terminations for the signal traveling in the wrong direction by using an absorbing material, such as wax or teflon, placed on the surface of the piezoelectric material to absorb the acoustic surface wave. This approach of course does not save any of the 6 db conversion loss.
A second approach to this problem, illustrated on page 1252 of the Proceedings of the IEEE, Vol. 58, No. 8, is to utilize as a launching transducer, two identical interdigital transducers separated by a distance (n + 1/4).lambda. where .lambda. is the surface wave wavelength and n is an integer. If these two transducers are supplied with electrical signals 90.degree. out of phase they will each cause surface waves to propagate in the piezoelectric material, however, surface waves traveling in one direction will add in phase while those traveling in the opposite direction will cancel. This approach increases the overall transducer conversion efficiency by 3 db since the resulting surface wave propagates in only one direction, however, it has a serious disadvantage of a very narrow bandwidth due to the large spacing between the two transducers utilized to launch the surface wave.